Code-17
Association denied; the access point is resource constrained
Code-18
Association denied;; the mobile station does not support all the data rates
required by the BSS
Code-19
Association denied; the mobile station does not support the short preamble option
Code-20
Association denied; the mobile station does not support PBCC modulation
Code-21
Association denied,, mobile station does notsupport channel agility
option

Will it related to frequency jam? not possible as during frequency JAM...there is no communication possible however for a situation to lead to association failure, STA need to do probing, clear authentication and then comes association stage.

or Will it related to the maxium number of client the AP supported?

yes, its possible. see Code-17
Association denied; the access point is resource constrained. AP's capacity to handle client is full or near full. this may happen.

Someone in ruckus can confirm how AP is expecting to behave if client limit reaches max

So will there any possible way to check it myself first by AP log or SCG log? Since at that time nearly 100 client on 2.4G and 100 client on 5G that concurrent using on the single AP Where can I get those log?